The present invention broadly relates to fuel vapor recovery systems used in internal combustion engines, and deals more particularly with a system providing fuel tank isolation during purging of vapors from a carbon storage canister.
Evaporative emission control systems employing fuel vapor recovery have become widely used in internal combustion engine powered vehicles to prevent evaporative fuel from being emitted from a vehicle""s gas tank into the atmosphere. Known evaporative emission control systems typically employ one or more carbon canisters coupled with the fuel tank and with the engine by vapor lines, and a series of valves to control the flow of fuel vapor between the fuel tank, the canister, and the engine. The canister, which is coupled to the fuel tank, uses an activated carbon bed for absorbing the hydrocarbons contained in the fuel. The canister is periodically purged by passing ambient air through the activated carbon which desorbs the hydrocarbons. The resulting air and hydrocarbon mixture is delivered through a purge valve to the intake manifold of the engine and is combined with the normal flow of fuel so as to be combusted in the engine. These systems also draw fuel vapor from the fuel tank during the purging process so that the vapors from the tank and those from the canister are combined before being drawn into the intake manifold. Pressure transducers are commonly used to sense vapor pressure at various points in the system in order to check the system for leaks, as well as to provide pressure information to an electronic engine controller that is used to operate the system""s valves.
Although known evaporative emission systems are relatively effective, they are nevertheless subject to further improvements that reduce atmospheric emissions of hydrocarbons derived from fuel vapors. One of the problems of known systems is the need for regulating the flow of vapors received by the intake manifold in response to changes in the vapor pressure, relative to the intake manifold vacuum. This problem is exacerbated in those fuel recovery systems where vapor is drawn directly from the fuel tank into the engine. When the fuel tank is not completely full, and there is a volume of empty space above the fuel in the tank, there is a tendency for the fuel to slosh around while the vehicle is moving, thereby generating additional fuel vapors in the tank, and raising the vapor pressure. This increase in vapor pressure increases the level of vapor flow to the intake manifold which then must be compensated for by the appropriately modulating vapor flow through the purge valve into the engine. However, with a rapid rise or fall in the vapor pressure within the fuel tank, it is not always possible to precisely modulate the flow of vapor into the intake manifold. As a result, the combined amount of fuel and vapor combusted in the engine is not controlled as precisely as would be desired. It would therefore be desirable to more precisely control the flow of fuel vapor into the intake manifold in order to achieve more efficient fuel combustion and reduce atmospheric emissions of hydrocarbons.
A general object of the invention is to provide an evaporative emission control system that isolates the fuel tank during normal engine operation so as to allow more precise control of the delivery of recovered fuel vapor to the intake manifold of an internal combustion engine. This object is achieved by a valve system that isolates the fuel tank from a purge line that delivers vapors stored in a carbon canister through a modulated purge valve into the engine""s intake manifold, during normal operation of a vehicle. During a refueling event however, the purge valve is closed and a main vapor control valve is opened that allows fuel vapor within the fuel tank to flow into the carbon canister. After refueling, while the engine is shut down, both valves are closed to prevent the flow of vapors between the tank, the canister and the engine.
Thus, according to one aspect of the invention, the evaporative emission control system provides fuel vapor to an internal combustion engine. The system includes a fuel storage tank in which fuel vapor is generated, a canister for storing fuel vapor generated within the tank, and a flow control arrangement that isolates vapors in the tank from the engine during normal operation of the engine. The flow control arrangement includes a first vapor line connecting the tank with the canister, a vapor control valve for controlling the flow of vapor through the first line, a second vapor line connecting the first line with the engine, and a purge valve for controlling the flow of vapor through the second line to the engine. First and second pressure sensors are provided for respectively sensing the fuel vapor pressure in the tank and in the canister. An electronic engine controller is employed to operate the valves, based in part on information provided by the first and second pressure sensors.
According to another aspect of the invention, a method is provided for controlling the flow of fuel vapor from a fuel tank to an internal combustion engine having an evaporative emission control system. The method includes the steps of establishing a flow of fuel vapor from a vapor storage canister to the engine, while concurrently prohibiting the flow of fuel vapor in a fuel tank to enter the canister or the engine. The method further includes the steps of, during a refueling event, prohibiting the flow of vapor into the engine, while establishing a flow of vapor from a fuel tank to the canister.
The above objects and other obvious features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken with the accompanying drawings.